I have count data of incidents per month, a couple years prior to an intervention and 1 year after. I want to use poisson regression to test if the intervention had a significant impact on the number of incidents. This is a quasi-experimental study in which the intervention was applied in a non-random way. There is only 1 zero in the data. The data has a seasonal pattern so I am wondering how to model this in the poisson, do I need to create a dummy variable? How best do I go about modeling the dummy variables (month, quarter)?